815M17 / EN353 Steel: Grade Overview and History
Key fact: 815M17 and EN353 are identical steels. EN353 is the withdrawn BS 970:1955 designation; 815M17 is the current BS 970:1991 name. Both remain in active use on engineering drawings worldwide.
815M17 is a low-carbon, nickel-chromium-molybdenum alloy steel standardized under BS 970:1991 for case-hardening applications. Its predecessor designation under the older BS 970:1955 system was EN353 — a name still widely cited on workshop drawings and purchase orders across the United Kingdom, India, Southeast Asia, and Australia.
The designation system encodes the alloy precisely: "815" identifies the nickel-chromium steel family under BS 970, "M" confirms that both chemical composition and mechanical properties are controlled to specification, and "17" reflects the nominal carbon content of approximately 0.17%. Together, these three elements tell a materials engineer everything they need to understand the grade before opening a data sheet.
EN353 and 815M17 are exactly the same steel. BS 970:1955 was formally withdrawn and superseded by BS 970:1991, which renamed EN353 to 815M17. When a drawing or purchase order references EN353, treat it as 815M17 and verify chemistry against the BS 970:1991 composition table to confirm compliance.
The defining engineering logic of 815M17 / EN353 is the case-hardening principle: a low-carbon core (0.14–0.20% C) that stays tough and shock-absorbent, paired with a carburized surface layer that is quenched to martensitic hardness of 58–62 HRC. Nickel (1.20–1.70%) raises the impact toughness of the core and enables through-hardening in heavier section sizes. Chromium (0.80–1.20%) improves hardenability and surface hardness response. Molybdenum (0.10–0.20%) suppresses temper embrittlement and extends hardenability into cross-sections above 75 mm — a critical advantage over simple Ni-Cr grades without Mo.
The result is a steel that excels precisely where single-element grades struggle: components that must simultaneously carry high cyclic contact loads at the surface and absorb sudden shock through the cross-section. Gear teeth, automotive transmission shafts, camshafts, and heavy-duty track pins are the canonical examples of this dual-performance demand.
Chemical Composition of 815M17 / EN353 per BS 970:1991
Composition summary: C 0.14–0.20% · Si 0.10–0.35% · Mn 0.60–0.90% · Cr 0.80–1.20% · Ni 1.20–1.70% · Mo 0.10–0.20% · P max 0.035% · S max 0.040%
The composition of 815M17 is intentionally lean in carbon. Surface carbon is added during the carburizing heat treatment process — it is not present in the base steel. This design keeps the core ductile, the forging process straightforward, and enables fine-grained microstructure control throughout the heat treatment cycle. For available forms, dimensional range, and to request a quote, see the 815M17 EN353 forging parts product page.
| Element | Symbol | Min % | Max % | Engineering Role in 815M17 |
|---|---|---|---|---|
| Carbon | C | 0.14 | 0.20 | Low C = tough, forgeable core. Surface carbon added by carburizing to reach 0.75–0.85% at the case. |
| Silicon | Si | 0.10 | 0.35 | Deoxidizer during steelmaking; minor solid-solution strengthener. |
| Manganese | Mn | 0.60 | 0.90 | Improves hardenability; counteracts sulfide embrittlement from sulfur. |
| Chromium | Cr | 0.80 | 1.20 | Increases hardenability; improves response to surface case hardening; refines carbide distribution. |
| Nickel | Ni | 1.20 | 1.70 | Core toughness; low-temperature impact resistance; hardenability in heavy-section forgings above 75 mm. |
| Molybdenum | Mo | 0.10 | 0.20 | Suppresses temper embrittlement; extends hardenability in thick-section forgings; improves fatigue resistance. |
| Phosphorus | P | — | 0.035 max | Controlled residual. Excess phosphorus causes grain-boundary embrittlement and reduces impact toughness. |
| Sulfur | S | — | 0.040 max | Controlled residual. Can be tightened to ≤ 0.010% for aerospace or critical gear forgings requiring premium cleanliness. |
Why the Ni-Cr-Mo balance is critical in large forgings
Nickel and chromium act synergistically in 815M17. Chromium alone improves hardenability but can cause core brittleness in heavy sections if not balanced by nickel. Nickel alone improves toughness but reduces surface hardness response. The molybdenum addition — at only 0.10–0.20% — plays a disproportionately important role in suppressing temper brittleness during the slow cooling phase of large cross-section forgings, and in maintaining adequate hardenability when section thickness exceeds 75–100 mm.
For aerospace, automotive transmission, or high-speed gearbox applications, additionally specify gas content limits (O₂ < 20 ppm, H₂ < 2 ppm, N₂ < 100 ppm), ASTM grain size 6 or finer, and macro-etch Class C/c or better per your applicable inspection standard. These limits are not in the standard BS 970:1991 specification and must be stated explicitly on the purchase order or attached material specification.
Mechanical Properties of 815M17 After Case Hardening
Key properties (CH+H+T condition): Surface hardness 58–62 HRC · Case depth 0.8–1.5 mm · Core tensile ≥ 770 N/mm² · Core yield ≥ 580 N/mm² · Charpy impact ≥ 55 J at 20°C · Annealed delivery hardness max 375 HB
815M17 / EN353 is always evaluated in its final carburized, hardened, and tempered (CH+H+T) condition. The base material in annealed or normalized condition has limited engineering significance — the properties that determine suitability for service are those achieved after the full case-hardening cycle in the finished part.
"For gear applications, specifying surface hardness alone is insufficient. Procurement engineers should always request effective case depth to 550 HV10, core hardness in HRC or HB, and Charpy impact at the test piece center — not just the surface reading on the MTC."
Surface vs. core: two distinct microstructures in one part
The case and core of a 815M17 component carry entirely different mechanical roles. The carburized case — typically 0.8–1.5 mm effective depth reaching up to 0.80% C at the surface — provides compressive residual stresses that significantly extend fatigue life under rolling contact and sliding wear. The core, at minimum 770 N/mm² tensile strength, provides the structural backbone that prevents tooth fracture, shaft bending failure, or pin shear under torque spikes and shock loads. It is this simultaneous optimization of two independent mechanical demands that makes 815M17 the dominant gear steel across industrial sectors worldwide. Buyers sourcing custom 815M17 EN353 forgings should specify both surface hardness and core Charpy impact requirements on the purchase order.
Section size effect on core properties
As with all hardenable steels, 815M17 exhibits a section size effect: core hardness and tensile strength decrease as section diameter increases because the cooling rate at the center of a large forging is slower. The molybdenum addition meaningfully mitigates this compared to a simple Ni-Cr grade without Mo. For forgings above 100 mm in cross-section, buyers should request Jominy hardenability data (J-curve) from the specific melt lot to verify adequate through-hardening before finalizing the material approval.
Heat Treatment Process for 815M17 / EN353 Forgings
Process overview: Soft anneal (680–710 °C) → rough machine → carburize (900–950 °C, 4–16 h) → oil quench → temper (150–200 °C) → finish grind & inspect.
815M17 / EN353 requires a two-stage heat treatment sequence: a soft-annealing step to prepare the forging for machining, followed by the production carburizing and hardening cycle that imparts the final surface and core properties. Each step is interdependent — skipping or misordering stages leads to distortion, dimensional rejection, or inadequate surface hardness.
Soft Annealing — delivery condition from the forge shop
The as-forged blank is annealed to reduce hardness for machining. The maximum annealed hardness is 375 HB. This step is performed at the forge facility before dispatch to the machining or gear-cutting operation.
680–710 °C · Furnace cool to ≤ 600 °C · Air cool · Max 375 HBRough Machining to Near-Net Shape
The forging is rough-machined leaving 0.15–0.25 mm stock per surface for final grinding after case hardening. Areas not to be carburized are protected by copper electroplating or commercial anti-carburizing paste applied to the relevant surfaces before furnace entry.
Carburizing — gas or vacuum process
Carbon diffuses into the surface in a carbon-rich atmosphere. Gas carburizing (endothermic atmosphere at carbon potential 0.75–0.85%) is the industry standard for industrial gear production. Vacuum low-pressure carburizing (LPC) is preferred for precision automotive or aerospace gears to eliminate intergranular oxidation and minimize distortion.
900–950 °C · 4–16 h · Carbon potential 0.75–0.85% · Case depth 0.8–1.5 mmQuenching — direct or re-heat
For most industrial forgings, the component is direct-quenched from carburizing temperature into oil (preferred) or polymer quenchant. Precision gears may be re-heated to a lower austenitizing temperature (820–840 °C) before quenching to refine the case grain size and reduce distortion on complex profiles.
820–870 °C → Oil quench · Surface: 58–62 HRC achievedTempering — stress relief
Low-temperature tempering reduces quench stresses and prevents case microcracking while fully retaining the case hardness. Core tempering at higher temperature would reduce case hardness to below target and is avoided unless a specific lower surface hardness is required by the design.
150–200 °C · 1–2 h · Air cool · HRC 58–62 fully retainedFinish Grinding and Final Inspection
Profile or cylindrical grinding removes the remaining stock and achieves final dimensional tolerance. Acceptance inspection includes surface hardness (HRC), effective case depth measurement (taper polish section or metallographic section), dimensional check per drawing, and Magnetic Particle Inspection (MPI) of gear teeth or critical fillets.
When ordering 815M17 forgings from an overseas supplier, always specify the required delivery condition explicitly in your purchase order: "Soft annealed, maximum 375 HB, EN 10204 3.1 Mill Test Certificate required." Final case hardening is almost always performed at your facility or a specialist heat treater — not the forge shop — unless specifically contracted as a value-added service.
International Grade Equivalents for 815M17 / EN353
Key equivalents: DIN 20NiCrMo5 · EN 10084: 17NiCrMoS6-4 (1.6566) · AFNOR 20NCD5 · SAE 4320 (closest, not exact) · GB/T 20CrNiMo · IS 15Ni2Cr1Mo28
815M17 / EN353 is a British Standard designation, but the same Ni-Cr-Mo case-hardening chemistry is produced globally under different national standards. Understanding these cross-references is essential when specifying material on international projects, converting drawings between specification systems, or sourcing from non-UK forge suppliers.
| Standard System | Grade Designation | Country / Region | Notes |
|---|---|---|---|
| BS 970:1991 | 815M17 | United Kingdom | Current active designation |
| BS 970:1955 | EN353 | United Kingdom | Withdrawn; widely cited on legacy drawings |
| DIN 17210 | 20NiCrMo5 | Germany | Very close composition match |
| EN 10084 / EN 10277-4 | 17NiCrMoS6-4 (1.6566) | Europe (harmonized) | Current European harmonized standard |
| AFNOR NF A35-552 | 20NCD5 / 20NCD5.5 | France | Good composition overlap |
| SS 14 2523 | SS 2523 | Sweden | Equivalent intent and hardenability |
| SAE J404 | 4320 (closest match) | USA | No exact equivalent; Ni range differs; verify properties independently |
| IS 4432 | 15Ni2Cr1Mo28 | India | Indian standard equivalent grade |
| GB/T 3077 | 20CrNiMo | China | Nearest Chinese equivalent |
There is no exact American AISI/SAE equivalent to 815M17. The closest grade is SAE 4320 per SAE J404 — a Ni-Cr-Mo low-carbon case-hardening steel sharing similar hardenability intent. However, SAE 4320 has a higher Ni range (1.65–2.00%) and different Cr window compared to 815M17. When converting a BS 970 design to US material, always independently verify that the substitute material meets the mechanical property requirements specified by the design authority. Grade cross-references are starting points for equivalence assessment — not substitution authorizations.
Forging Characteristics of 815M17 / EN353 Steel
815M17 / EN353 is an excellent forging alloy. Its low carbon content keeps forging temperatures accessible and avoids the grain-growth problems that higher-carbon gear steels introduce at elevated temperatures. The following characteristics define how forge shops approach this material in production.
Hot forging temperature range
The optimal hot forging range for 815M17 is 1,050–1,250 °C, with a recommended finish forging temperature no lower than 950 °C. Finishing above 950 °C ensures a uniform, fine grain structure and prevents the formation of a duplex (mixed fine and coarse grain) microstructure that would impair fatigue properties and cause certification failure on metallographic inspection. Below 900 °C, work-hardening accelerates rapidly and the risk of surface cracking increases significantly. After forging, parts are air-cooled or placed in an insulated pit for controlled slow cooling — they must never be direct-quenched from forging heat, especially in heavy sections where rapid hydrogen evolution can cause cold cracking.
Grain size control for gear applications
For gearing and precision transmission applications, ASTM grain size 6 or finer is typically specified per ASTM E112. 815M17 achieves this reliably when finish forging is performed at temperatures ≤ 1,050 °C with a reduction ratio of at least 4:1 from the ingot or billet cross-section. Grain refinement is confirmed by metallographic examination of a polished and etched test coupon, taken from a full-size forging of the same heat and heat treatment lot.
Available forging forms — Jiangsu Liangyi's 815M17 product range
As a specialist forging manufacturer, Jiangsu Liangyi Co., Limited produces 815M17 / EN353 in the following forms to customer drawings and dimensional specifications:
- Open die forged round bars — Ø 60 mm to 1,200 mm diameter, lengths to 8,000 mm
- Forged gear blanks and discs — outside diameter to 2,500 mm, single-piece weight to 30,000 kg
- Seamless rolled forged rings — OD up to 5,000 mm, wall thickness from 50 mm
- Forged shafts and step shafts — transmission input/output shafts, pinion shafts, gear shafts, spindles
- Hollow forgings and sleeves — for spindles, hubs, bearing journals, and hydraulic cylinder barrels
All 815M17 / EN353 forgings are produced from a single melt heat for full chemistry traceability. Delivery is in the soft-annealed condition (max 375 HB). EN 10204 Type 3.1 mill test certificates are available on request. Please specify your documentation requirements at the time of enquiry.
Industrial Applications of 815M17 / EN353 Forgings
Primary applications: Industrial gears · Automotive transmission shafts · Camshafts · Track pins · Steering pivots · Mining drive shafts · Wind turbine nacelle components
815M17 / EN353 is the dominant case-hardening forging grade wherever a component must combine a hard wear-resistant surface with a tough, shock-absorbing core. The following application families account for the majority of global 815M17 forging consumption.
Industrial Gears & Gear Ring Blanks
Spur, helical, and bevel gear blanks for power generation, cement mill, paper mill, and steel mill gearboxes. Forged blanks ensure favorable grain flow alignment for maximum bending fatigue resistance at the tooth root fillet.
Automotive Transmission Shafts
Input shafts, layshafts, pinions, and synchronizer hubs for passenger car and commercial vehicle gearboxes. EN353's toughness and surface hardness combination directly matches automotive OEM case-depth and core strength specifications.
Camshafts & Valve Gear
Engine camshafts, cam followers, and valve rockers where the lobe surface requires hardness ≥ 58 HRC and the shaft body must resist bending fatigue over hundreds of millions of cycles. Low-distortion heat treatment suits precision cam profiles.
Track Pins & Crawler Undercarriages
Construction and mining crawler track pins experience extreme abrasion at the outer diameter and high shear loads at the bore interface. 815M17 case-hardened pins deliver the surface and core balance required for extended service life in these severe conditions.
Steering & Suspension Pivots
King pins, ball pin journals, and rack shafts in commercial vehicle steering systems rely on EN353 for wear resistance at the bearing contact surface and core toughness to survive road shock loads and misalignment moments.
Mining Equipment Drive Shafts
High-torque drive shafts in crushers, conveyors, and ore processing equipment, where cyclic fatigue from variable motor loads demands the combination of fatigue limit and notch toughness that case-hardened 815M17 reliably delivers.
815M17 vs. Similar Case-Hardening Steel Grades
Selection rule of thumb: Choose 815M17 for sections up to ~100 mm and core tensile below 900 N/mm² — it delivers the best value/performance balance. Step up to EN36 / 832M13 only for heavier sections or aerospace/defense duty levels requiring >1,000 N/mm² core tensile.
Selecting between 815M17 and competing case-hardening grades requires balancing section size, required core toughness, target case depth, heat treatment distortion sensitivity, and procurement cost and availability. The table below provides a direct engineering comparison of the most commonly considered alternatives:
| Grade | C % | Ni % | Cr % | Mo % | Core Tensile (min) | Best Fit |
|---|---|---|---|---|---|---|
| 815M17 / EN353 | 0.14–0.20 | 1.20–1.70 | 0.80–1.20 | 0.10–0.20 | 770 N/mm² | General gearing, medium sections ≤100 mm; best value |
| 655M13 / EN36 | 0.10–0.16 | 3.90–4.30 | 1.10–1.40 | — | 1,080 N/mm² | Heavy sections, high-duty industrial gears |
| 832M13 / EN36C | 0.10–0.16 | 3.90–4.30 | 1.10–1.40 | 0.20–0.35 | 1,155 N/mm² | Aerospace, thick forgings, critical duty |
| 527M20 / EN34 | 0.17–0.23 | 1.20–1.70 | 0.90–1.30 | — | 700 N/mm² | Thin sections (< 50 mm) where Mo hardenability is marginal |
| SAE 8620 (AISI) | 0.18–0.23 | 0.40–0.70 | 0.40–0.60 | 0.15–0.25 | ~760 N/mm² | North American sourcing; lower Ni content reduces cost |
When to choose 815M17 over EN36 / 832M13
EN36 and 832M13 (both with approximately 4% Ni) provide significantly higher core strength and deeper hardenability, making them the correct choice for large-section or heavily loaded aerospace and defense gearing. However, the substantially higher nickel content (nearly 3× the nickel in 815M17) drives material cost up considerably. 815M17 / EN353 is the economically superior choice for section sizes below approximately 100 mm in diameter and where core tensile requirements do not exceed 900 N/mm² — which covers the great majority of industrial gearbox, mining equipment, and automotive transmission forging applications.
When 527M20 / EN34 can substitute for 815M17
527M20 (EN34) is essentially 815M17 without the molybdenum addition. For section sizes below 50 mm, where the Mo contribution to hardenability is marginal, 527M20 provides virtually identical service properties at marginally lower alloy cost. For heavier sections, for applications involving elevated temperature service (where Mo suppresses temper embrittlement), or where an extended hardenability guarantee is contractually required, 815M17 is clearly the correct choice. View our full range of available forms and sizes on the 815M17 EN353 forging parts product page.
Sourcing & Quality Checklist for 815M17 / EN353 Forgings
When placing a purchase order for 815M17 / EN353 forgings from a global supplier, the following eight requirements must be explicitly stated to ensure receipt of compliant, fully traceable material that meets your design intent:
| # | Requirement | What to State on the Purchase Order |
|---|---|---|
| 1 | Material Standard | BS 970:1991, Grade 815M17 (or EN353 for legacy drawings — state which takes precedence) |
| 2 | Melting Route | EAF + LF + VD minimum for industrial grades; additionally specify ESR for premium cleanliness or aircraft-quality requirements |
| 3 | Chemistry Limits | Full BS 970:1991 composition including P ≤ 0.035%, S ≤ 0.040% (tighten S to ≤ 0.010% for critical gear forgings) |
| 4 | Grain Size | ASTM grain size 6 or finer per ASTM E112. State test frequency: per heat lot or per individual piece for premium orders |
| 5 | Delivery Hardness | Soft annealed, maximum 375 HB. State Brinell hardness test per forging or per lot as applicable |
| 6 | Ultrasonic Testing | EN 10228-3 Class 3 or ASTM A388 Class C. State the applicable standard and acceptance class explicitly |
| 7 | Mill Test Certificate | EN 10204 Type 3.1 MTC available on request; third-party witnessed documentation can be arranged — specify at time of enquiry |
| 8 | Identification Marking | Heat number, material grade, and applicable standard stamped or cold-stamped on each piece, or on a permanently attached tag per MTC |
Jiangsu Liangyi Co., Limited works to satisfy all eight items on this checklist for every 815M17 / EN353 forging order. Our ISO 9001:2015 quality management system covers the complete production chain from steel melting through final inspection and dispatch. EN 10204 3.1 mill test certificates are available on request for every order; full heat traceability records are retained for a minimum of 10 years.
Frequently Asked Questions about 815M17 / EN353 Steel
Yes. EN353 is the BS 970:1955 designation; 815M17 is the current BS 970:1991 name for the exact same nickel-chromium case-hardening steel. When BS 970:1955 was formally withdrawn and replaced by BS 970:1991, EN353 was renamed to 815M17 within the new designation system. Both names remain in active use on engineering drawings, purchase orders, and material specifications worldwide — particularly in the UK, India, Southeast Asia, and Australia.
Per BS 970:1991, 815M17 contains: Carbon 0.14–0.20%, Silicon 0.10–0.35%, Manganese 0.60–0.90%, Chromium 0.80–1.20%, Nickel 1.20–1.70%, Molybdenum 0.10–0.20%, Phosphorus maximum 0.035%, Sulfur maximum 0.040%. It is classified as a 1.25% nickel-chromium low-carbon case-hardening alloy steel.
There is no exact AISI/SAE equivalent to 815M17. The closest American grade is SAE 4320 per SAE J404, which shares the low-carbon, Ni-Cr-Mo case-hardening intent. However, SAE 4320 has a higher Ni range (1.65–2.00%) and a different Cr window compared to 815M17. When converting a BS 970 design to US material, always independently verify that SAE 4320 meets the design's mechanical property requirements — grade cross-references are not substitution authorizations.
In the carburized, hardened, and tempered condition: surface hardness 58–62 HRC; effective case depth 0.8–1.5 mm to 550 HV10; core tensile strength minimum 770 N/mm²; core yield strength minimum 580 N/mm²; Charpy impact energy ≥ 55 J at 20°C. Annealed delivery hardness maximum 375 HB. These values are for standard section sizes — core properties decrease in sections above 100 mm diameter.
Yes, with appropriate precautions. The low carbon content (0.14–0.20% C) gives 815M17 acceptable weldability in the annealed condition. Preheat to 150–200 °C and use a matched Ni-Cr-Mo filler wire. Forgings that have already been case-hardened and tempered should not be welded — welding heat input will destroy the carefully developed case microstructure in the heat-affected zone, creating a soft spot at what is typically the highest-loaded surface of the component.
For standard open die forged bars and gear blanks in 815M17, the typical lead time from order confirmation to dispatch is 20–35 working days. Complex near-net-shape machined components, or very heavy single pieces above 10,000 kg, may require 40–55 working days. Contact our sales team with your drawing and quantity for a firm quotation and committed delivery schedule.
Jiangsu Liangyi accepts a minimum order of 30 kg per specification and forging form. This means prototype and sample orders are fully supported with the same ISO 9001:2015 quality controls and EN 10204 3.1 documentation as full production volume orders. There is no price penalty for small quantities beyond the standard per-kg rate for that weight bracket.
Source 815M17 / EN353 Forgings from a Verified ISO Manufacturer
Submit your drawing or specification and receive a detailed free quotation within 24 hours. ISO 9001:2015 certified · EN 10204 3.1 MTC available on request · 50+ export countries · 25+ years' experience · MOQ 30 kg.